System and method for determining atrioventricular pacing delay based on atrial depolarization

ABSTRACT

Techniques are provided for estimating optimal atrioventricular pacing delay values for use in pacing the ventricles based on features of an intracardiac electrogram (IEGM) signal. Briefly, atrioventricular pacing delay pacing values are set based upon the location of atrial repolarization events within the IEGM. In one example, the end of an atrial repolarization is identified, then the interval from the atrial depolarization to the end of the atrial repolarization is measured. The atrioventricular pacing delay is then set by subtracting an offset value from that interval so as to time delivery of V-pulses prior the end of atrial repolarization. In this manner, atrioventricular pacing delay values are set based only IEGM signals and hence can be set to optimal/preferred values by the device itself without requiring surface electrocardiogram (EKG) signals and Doppler echocardiography or other cardiac performance monitoring techniques.

FIELD OF THE INVENTION

The invention generally relates to implantable cardiac stimulationdevices for use in pacing the heart of a patient and in particular totechniques for determining optimal or preferred atrioventricular pacingdelay times for individual patients based on intracardiac electrogram(IEGM) signals.

BACKGROUND OF THE INVENTION

A pacemaker is implantable cardiac stimulation device for implant withina patient that analyzes an IEGM to detect various arrhythmias such as anabnormally slow heart rate (bradycardia) or an abnormally fast heartrate (tachycardia) and delivers electrical pacing pulses to the heart inan effort to remedy the arrhythmias. An implantablecardioverter-defibrillator (ICD) additionally detects atrialfibrillation (AF) or ventricular fibrillation (VF) and deliverselectrical shocks to terminate fibrillation.

For many patients, particularly those with congestive heart failure(CHF), it is desirable to identify a set of control parameters forcontrolling the operation of the pacemaker or ICD that will yieldoptimal cardiac performance (also referred to as hemodynamicperformance). Cardiac performance is a measure of the overalleffectiveness of the cardiac system of a patient and is typicallyrepresented in terms of stroke volume or cardiac output. Stroke volumeis the amount of blood ejected from the left ventricle during systole ina forward direction. Cardiac output is the volume of blood pumped by theleft ventricle per minute (or stroke volume times the heart rate). Inview of the importance of maintaining optimal cardiac performance,especially for patients with compromised cardiac function, it would bedesirable to provide improved techniques for use with pacemakers or ICDsor other implantable cardiac stimulation devices for identifying pacingcontrol parameters that optimize cardiac performance, particularly toreduce the degree of heart failure and valvular regurgitation. It is tothis end that aspects of the invention are generally directed.

A useful control parameter for optimizing cardiac performance is theatrioventricular pacing delay, referred to herein is the A-VP delay,which for dual chamber devices specifies the time delay between a pacedor sensed atrial event and a paced ventricular event. Sensed events(i.e. intrinsic or native events) are also referred to as depolarizationevents as these events are representative of electrical depolarizationof myocardial tissue. Paced events are also referred to herein as evokedresponses. Paced events in the atria are triggered by A-pulses generatedby the implantable device. Paced events in the ventricles are triggeredby V-pulses also generated by the implantable device. Note that, herein,“A” is generally used to refer to atrial events, whether paced orsensed. “V” is used to generally refer to ventricular events, whetherpaced or sensed. In circumstances where it is necessary to distinguishbetween paced and sensed events, an “S” or “P” is appended. Hence, ASrefers to a sensed atrial event, whereas AP refers to paced atrialevent. VS refers to a sensed ventricular event, whereas VP refers to apaced ventricular event. Thus, A-VP generally represents the delaybetween either a paced or sensed atrial event and a paced ventricularevent. AS-VP specifically refers to the delay between a sensed atrialevent and the paced ventricular event; whereas AP-VP specifically refersto the delay between a paced atrial event and the paced ventricularevent.

In addition, where appropriate, an “L” or “R” subscript is employedherein to distinguish between the left and right chambers of the heart.For example, AP_(R) refers to a paced event in the right atrium. VS_(R)refers to a sensed event in the right ventricle. Hence, AP_(R)-VS_(R)represents the delay between a paced event in the right atrium and asensed event in the right ventricle. Also, where appropriate, a “PEAK”or “END” subscript is employed herein to distinguish between the peakand end of a given event. For example, AS_(PEAK) represents the peak ofa sensed atrial event; whereas AS_(END) represents the end of the sensedatrial event. The term “intrinsic delay”, as used herein, refers to thedelay between a paced or sensed event in one chamber and a subsequentsensed depolarization in another chamber. For example, an “intrinsicatrioventricular delay” refers to the delay between a paced or sensedatrial event and a subsequent sensed ventricular event, e.g. an AS-VS orAP-VS delay. Also, a “T” is used herein to identify repolarizationevents. For example, AT refers to an atrial repolarization; whereas VTrefers to a ventricular repolarization. Note that the A, V and T events,whether paced or sensed, are all features of the IEGM signal sensed andrecorded by the implantable device. The features are also observable insurface electrocardiogram (EKG) signals obtained via leads temporarilyaffixed to the chest of the patient. The corresponding feature of an ASevent observed within the surface EKG is referred to as a P-wave. Thecorresponding feature of a VS event observed within the surface EKG isreferred to as an R-wave. The corresponding feature of a repolarizationevent observed within the surface EKG is referred to as a T-wave.Finally, note that the VS event of the IEGM is also often referred to asa QRS complex.

In normal patients, the electrical conduction through theatrioventricular node is intact, and the body automatically adjusts theintrinsic atrioventricular delay (AS-V) via the circulating hormones andthe autonomic nervous system according to its physiologic state. It iswell known, for example, that in normal patients the intrinsicatrioventricular delay shortens with increasing heart rate associatedwith a physiologic stress such as exercise. For patients with abnormalatrioventricular node conduction or complete heart block, a pacemakercan control the A-VP delay (i.e. either the AS-VP delay, the AP-VP delayor both) by delivering a ventricular pacing pulse at asoftware-controlled delay after an atrial pace or atrial sensed event.Since the optimum A-VP delay varies from person to person, thisparameter should be optimized on an individual basis.

Conventionally, the physician attempts to program the A-VP delay (orother parameters) for a given patient by using an external programmer tocontrol the device implanted within the patient to cycle through a setof different A-VP delay values. For each value, the implanted devicepaces the heart of the patient for at least a few minutes to permithemodynamic equilibration, then the physician records a measure of theresulting cardiac performance, measured, for example, using Dopplerechocardiography. The A-VP delay value that yields the best cardiacperformance is then selected and programmed into the device. However,this is a time consuming and potentially expensive procedure. As aresult, some physicians do not bother to optimize A-VP delay in many oftheir patients. Rather, A-VP delay is merely set to a default value andis adjusted only if the patient does not respond well to pacing therapyor complains that they do not feel well.

Hence, many patients are not paced at their particular optimal A-VPdelay value and thus do not obtain the maximal potential benefit fromthe improved cardiac performance that could otherwise be gained.Moreover, even in circumstances wherein A-VP delay is optimized by thephysician using, for example, Doppler echocardiography, the time andassociated costs are significant. In addition, the optimal A-VP delayfor a particular patient may change with time due to, for example,progression or regression in CHF, changes in medications, and/or changesin overall fitness. However, with conventional optimization techniques,the A-VP delay is re-optimized, if at all, only during speciallyscheduled follow-up sessions with the physician to allow access to thenoninvasive testing equipment such as Doppler-echocardiography, whichsessions may be months or perhaps years apart.

Accordingly, it is would be highly desirable to provide improvedtechniques for more easily and reliably determining optimal or otherwisepreferred A-VP delay values for a particular patient. Preferably, suchtechniques would be designed so as to be performed by the implantabledevice itself using only IEGM data, so that Doppler echocardiography orother expensive and time consuming cardiac performance monitoringtechniques are no longer required. This permits the optimal A-VP delayto be frequently and automatically updated so as to respond to changeswithin the patient.

Many of these needs have been met by techniques set forth in U.S. patentapplication Ser. No. 10/928,586, of Bruhns et al., entitled “System andMethod for Determining Optimal Atrioventricular Delay based on IntrinsicConduction Delays”, filed Aug. 27, 2004, which is incorporated byreference herein. Briefly, techniques are provided therein where boththe intrinsic inter-atrial conduction delay and the intrinsicatrioventricular conduction delay are determined for the patient andthen preferred A-VP delay values are derived therefrom. In one example,the technique uses only IEGM signals and surface EKG signals and hencecan be performed by an external programmer without requiring Dopplerechocardiography or other cardiac performance monitoring techniques. Inanother example, wherein the implanted device is equipped with acoronary sinus lead, the technique uses only IEGM signals and hence canbe performed by the device itself.

Although the techniques of Bruhns et al. are effective, it would bedesirable to provide alternative techniques for determining optimal orotherwise preferred A-VP delay values for a patient, particularlytechniques that do not require the use of a surface EKG, and it is tothat end that the present invention is more specifically directed.

SUMMARY

In accordance with one illustrative embodiment, techniques are providedfor determining atrioventricular pacing delay values (i.e. A-VP values)for use in delivering cardiac pacing therapy to the heart of a patientin which an implantable cardiac stimulation device is implanted.Broadly, an atrial repolarization event is detected within an electricalcardiac signal and then A-VP values are determined based on the atrialrepolarization event. By basing the determination of the A-VP delayvalues on atrial repolarization events, pacing delay values can bereadily determined based only on IEGM signals so that surface EKGsignals need not be employed and cardiac performance monitoringtechniques, such as Doppler echocardiography, are not required.

Preferably, separate A-VP delay values are determined for use with pacedatrial events and sensed atrial events, i.e. separate values aredetermined for AS-VP pacing and for AP-VP pacing. In one example, theAS-VP delay is set relative to the end of the atrial repolarizationevent (AT_(END)) by determining the duration of an AS_(PEAK)-AT_(END)interval, then subtracting a predetermined offset value(offset_(SENSED)) suitable for use with sensed (i.e. intrinsic) atrialevents. (Alternatively, rather than timing the interval beginning withAS_(PEAK), the interval may be timed beginning at the point when the ASis first detected.) The AS-VP delay is then used to time the delivery ofV-pulses following detection of intrinsic atrial depolarization events(i.e. P-waves). The AP-VP delay is also set relative to the end of theatrial repolarization event (AT_(END)) by determining the duration of anAP_(PEAK)-AT_(END) interval, then subtracting a different predeterminedoffset value (offset_(PACED)) suitable for use with paced atrial events.(Alternatively, rather than timing the interval beginning withAP_(PEAK), the interval may be timed beginning at the point at which theA-pulse is delivered.) The AP-VP delay is then used to time the deliveryof V-pulses following paced atrial depolarization events (i.e.A-pulses). In one specific example, offset_(PACED) is set to 10 ms andoffset_(SENSED) is set to 20 ms.

It is believed that the AS-VP and AP-VP delay values calculated in thismanner approximate optimal delay values in that the values tend tomaximize ventricular filling so as to maximize cardiac performance.However, even if the delay values differ from true optimal values, theynevertheless represent preferred delay values likely to improveventricular filling. Preferably, delay values for both AS-VP and AP-VPare calculated and used. Alternatively, a preferred AS-VP value could becalculated and used in conjunction with an AP-VP value selected usingotherwise conventional techniques, or vice versa.

By calculating the AS-VP and AP-VP delay values by subtracting offsetsfrom the measured intervals (i.e. AS_(PEAK)-AT_(END), andAP_(PEAK)-AT_(END), respectively), the V-pulses are thereby timed to bedelivered before the end of subsequent atrial repolarization events.Hence, the ends of the atrial repolarization events are no longerdetectable within the IEGM. Preferably, ventricular pacing isperiodically suspended to again allow for detection of the ends ofatrial repolarization events so that the AS-VP and AP-VP delay valuescan be reset if needed. Also preferably, steps are performed to verifythat the intrinsic atrioventricular delay of the patient (i.e. the AS-VSand AP-VS delays) are sufficiently long so that AT_(END) can be reliablydetected. In one example, ventricular pacing is suspended to allowdetection of the AS-VS and AP-VS delays, which are then compared againstminimum atrial repolarization detection threshold values. So long as theAS-VS and AP-VS delays exceed their respective threshold values, thereis a sufficient interval of time to allow the ends of atrialrepolarization events to be detected so as to permit optimizing theAS-VP and AP-VP delay values based on the timing of those ends.Otherwise, alternative optimization techniques are preferably employedfor setting the AS-VP and AP-VP delay values, which do not rely ondetection of the ends of the atrial repolarization events.

Depending upon the implementation and the circumstances, the AS-VP andAP-VP delay values may be adjusted based on the intrinsic heart rate ofthe patient or on the current pacing rate. In one example, adjustment ofthe AS-VP delay value is performed by applying a scaling factor(β_(SENSED)) to an AS-VP delay value measured at patient rest rate,where the scaling factor is β_(SENSED)=(Current Intrinsic HeartRate)/(Patient Rest Rate). Adjustment of the AP-VP delay value isperformed by applying a scaling factor (β_(PACED) to an AP-VP delayvalue measured at a base pacing rate, where the scaling factor isβ_(PACED)=(Current Pacing Heart Rate)/(Base Pacing Rate).

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further features, advantages and benefits of the inventionwill be apparent upon consideration of the present description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a flow chart providing an overview of techniques provided inaccordance with the invention for identifying preferred A-VP delayvalues based on atrial repolarization (AT) events;

FIG. 2 is a flow chart illustrating an exemplary implementation of thetechnique of FIG. 1 wherein ends of atrial repolarization events areused to determine preferred A-VP delay values;

FIG. 3 is a flow chart illustrating an exemplary technique for verifyingthat the ends of the atrial repolarization events can be detected foruse with the technique of FIG. 2;

FIG. 4 is a graph providing a stylized representation of an IEGM signal,particularly illustrating a threshold used by the technique of FIG. 3 toverify that the ends of atrial repolarization events can be detected;

FIG. 5 is a flow chart illustrating an exemplary technique fordetermining separate atrioventricular delay values for paced and sensedevents for use with the technique of FIG. 2;

FIG. 6 is a graph providing a stylized representation of an IEGM signal,particularly illustrating an offset used by the technique of FIG. 5 forsetting atrioventricular delay values relative to the ends of atrialrepolarization events;

FIG. 7 is a flow chart illustrating an exemplary technique for adjustingatrioventricular delay values based on heart rate or pacing rate for usewith the technique of FIG. 2;

FIG. 8 is a simplified diagram illustrating an implantable stimulationdevice for use in implementing the techniques of FIGS. 1-7; and

FIG. 9 is a functional block diagram primarily illustrating internalcomponents of the implantable device of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description includes the best mode presently contemplatedfor practicing the invention. This description is not to be taken in alimiting sense but is made merely to describe general principles of theinvention. The scope of the invention should be ascertained withreference to the issued claims. In the description of the invention thatfollows, like numerals or reference designators will be used to refer tolike parts or elements throughout.

Overview of Techniques for Determining A-VP Delay Values

Within FIG. 1, at step 100, atrial repolarization events are detectedwithin an electrical cardiac signal, such as an IEGM signal, sensedwithin the heart of a patient by an implantable medical device implantedwithin the patient. Atrial repolarization events, which represent theelectrical repolarization of myocardial tissue subsequent to a previousdepolarization of the tissue, are also referred to herein as atrialT-waves. Then, at step 102, preferred atrioventricular pacing delayvalues (i.e. A-VP values) are determined by the implantable device basedon the atrial repolarization events for use in pacing the heart of thepatient. By basing the determination of the A-VP delay values, in part,on atrial repolarization events, preferred or optimal A-VP delay valuescan be readily determined by the implantable device itself withoutrequiring complicated conventional techniques such as the use of Dopplerechocardiography and the like and without requiring the use of surfaceEKGs, and hence costs are reduced. Moreover, since the determination isperformed by the implanted device itself, the preferred or optimal A-VPdelay values can thereby be recalculated as often as needed, based uponnewly sensed IEGM signals, to update the A-VP delay values to maintainthem at preferred or optimal values. Hence, the patient benefits fromimproved cardiac performance.

Atrial Repolarization End-Based Example

Referring now to FIG. 2, an exemplary technique will now be describedthat determines the optimal/preferred A-VP delay values based, in part,on the ends of the atrial repolarization events. Beginning at step 200,the implanted device senses unipolar atrial and ventricular IEGM signals(i.e. A-IEGM and V-IEGM signals) via leads implanted in the heart. By“unipolar”, it is meant that the housing of the implanted device is usedas a return electrode in combination with a sensing electrode implantedwithin the heart. Preferably, a lead implanted within the right atriumis used to sense the unipolar A-IEGM signal; whereas a lead implantedwithin the right ventricle is used to sense the unipolar V-IEGM signal.

At step 202, the implanted device verifies that the ends of atrialrepolarization events can be detected within the unipolar A-IEGM, i.e.that the intrinsic atrioventricular delay within the heart of thepatient is sufficiently long such that the AT_(END) is not obscured byintrinsic ventricular depolarization events. Verification may beaccomplished using a technique to be described below with reference toFIGS. 3-4. If the ends of atrial repolarization events cannot bedetected within the unipolar A-IEGM signal (either due to shortintrinsic atrioventricular delays or for any other reason), thenalternative atrioventricular pacing delay optimization techniques mayinstead be used to set the A-VP delay values. Suitable alternativetechniques include, for example, techniques set forth in theabove-referenced patent application of Bruhns et al. Alternatively, A-VPdelay values obtained via the invention at another heart rate (such as abase/rest rate) may be rate adjusted, then applied at the current rate.

Assuming that the ends of the atrial repolarization events can bedetected, then, at step 204, the implanted device determines preferredatrioventricular pacing delay (A-VP) values based on intervals betweenthe peaks of atrial depolarization events and the ends of the atrialrepolarization events, using techniques described below with referenceto FIGS. 5-6. Preferably, separate atrioventricular pacing delay valuesare determined for use with sensed and paced atrial events, i.e. bothAS-VP and AP-VP values are determined. If the preferred AS-VP and AP-VPdelay values are to be used at a heart rate significantly different fromthe rate at which the delay values were calculated, then the delayvalues are preferably adjusted, at step 206, based upon the currentheart rate or pacing rate using techniques described below withreference to FIG. 7. Finally, at step 208, the implanted device pacesthe ventricles of the patient using the rate-adjusted AS-VP and AP-VPdelay values.

Turning now to FIGS. 3-4, the verification technique of step 202 of FIG.2 will now be described in detail. Processing depends upon whetheratrial events are sensed or paced, i.e. processing depends upon whetherthe implanted device is currently allowing the atria to beat on theirown or whether A-pulses are being delivered to the atria via theimplanted leads. Referring first to sensed events, beginning at step210, the implanted device detects the peak of an intrinsic atrialdepolarization within the A-IEGM signal (i.e. AS_(PEAK).) At step 212,the implanted device then detects the peak of the subsequent intrinsicventricular depolarization event within the V-IEGM signal (i.e.VS_(PEAK).) Alternatively, the A-IEGM signal may be used to detect boththe atrial depolarization peak and the ventricular depolarization peak.Otherwise conventional techniques may be employed for detectingdepolarization events within electrical cardiac signals and foridentifying the peaks of those events.

At step 214, the implanted device then determines the intrinsicatrioventricular delay interval for sensed atrial events by calculatingVS_(PEAK)-AS_(PEAK). The interval is then compared, at step 216, againsta minimum intrinsic atrial repolarization detection threshold that maybe set, for example, to 200 milliseconds (ms). If the interval exceedsthe threshold, then the intrinsic atrioventricular delay of the patientis sufficiently long to allow detection of the ends of intrinsic atrialrepolarization events, step 218, and processing returns to FIG. 2 fordetermination of preferred/optimal AS-VP delay values based upon theends of the atrial repolarization events. If, however, the interval doesnot exceed the threshold, then the intrinsic atrioventricular delay ofthe patient is not sufficiently long to allow for detection of the endsof the intrinsic atrial repolarization events, step 220. Accordingly,alternative optimization techniques are preferably employed to set theAS-VP delay, or the AS-VP delay is set to a default value.

Also, rather than detect the peak of the sensed atrial depolarizationevent at step 210, the device can instead use the time at which theatrial depolarization is first detected by the pacer/ICD. (Typically, ansensed atrial event is detected by comparing the A-IEGM signal against adetection threshold.) The intrinsic atrioventricular delay for sensedatrial events is then calculating as VS_(PEAK)-A-detected. This intervalis then compared against a suitable detection threshold, which maydiffer from the 200 ms detection threshold used when measuring theintrinsic atrioventricular delay peak-to-peak.

The minimum detection threshold is illustrated in FIG. 4, which providesa stylized representation of an exemplary heartbeat of the patientobserved within an A-IEGM signal, particularly illustrating an atrialdepolarization event (i.e. AS) 222, an atrial repolarization event (i.e.AT) 224, a ventricular depolarization event (i.e. VS or QRS-complex)226, and a ventricular T-wave 227 (i.e. VT). As can be seen, theQRS-complex does not begin until well after the end of the atrial T-waveand hence does not obscure the end of the atrial T-wave. This isverified by the implanted device by comparing the duration of thepeak-to-peak interval (i.e. VS_(PEAK)-AS_(PEAK)) against the minimumdetection threshold for use with sensed atrial events. If thepeak-to-peak interval were instead less than the threshold, thenportions of the QRS-complex might obscure the end of atrialrepolarization event, preventing its detection.

Note that, when the intrinsic atrioventricular delay is measuredpeak-to-peak as in FIG. 4, care must be taken in setting the minimumdetection threshold to account for those portions of the QRS complexpreceding the peak of the QRS-complex. Otherwise, the measured intrinsicatrioventricular delay might exceed the minimum threshold, even thoughinitial portions of the QRS-complex might obscure the end of atrialrepolarization events. The exemplary threshold value noted above of 200ms is typically sufficient to take this factor into account. Dependingupon the implementation, the detection threshold is reprogrammable bythe physician using an external programmer so that the physician canensure that the detection threshold is set to a proper value for usewith each particular patient. Alternatively, the intrinsicatrioventricular delay could be measured based on the start of theQRS-complex, rather than its peak. However, it is easier and morereliable to detect the peak of an event than to detect start of theevent and so, in the preferred implementation of the invention, theintrinsic atrioventricular delay is measured peak-to-peak.

Also note that, within a given electrical event, one or more peaks canbe identified. Within the example FIG. 4, the peak of the atrialdepolarization event is identified as the first maxima within the eventwhich, in the example, has a positive value (relative to a baselinesignal level.) A second maxima occurs within the atrial depolarizationevent which, in the example, has a negative value. The second maxima hasa larger absolute value than the first maxima. If desired, the secondmaxima could instead be used to define the peak of atrial depolarizationevent, with the detection threshold set accordingly. In general, any ofa variety of markers may be used within the IEGM signals to measurevalues representative of the intrinsic atrioventricular delay of thepatient for use in comparing against one or more thresholds to verifythat the end of the atrial repolarization can be reliably detected.

Returning to FIG. 3, the processing steps for use with paced atrialevents will now briefly be summarized. Beginning at step 228, theimplanted device detects the peak of a paced atrial depolarizationwithin the A-IEGM signal, i.e. the device detects the peak of the evokedresponse triggered in the atria by an A-pulse. The peak of the pacedatrial depolarization as sensed within the atria is referred to hereinas AP_(PEAK). At step 230, the implanted device detects the peak of thesubsequent intrinsic ventricular depolarization event within the V-IEGMsignal. Alternatively, the A-IEGM signal may again be used to detectboth the atrial depolarization peak and the ventricular depolarizationpeak. At step 232, the implanted device then determines the intrinsicatrioventricular delay interval for paced atrial events by calculatingVS_(PEAK)-AP_(PEAK). The duration of this interval is then compared, atstep 234, against a minimum paced atrial repolarization detectionthreshold set, for example, to 260 milliseconds (ms). If the intervalexceeds the minimum threshold, then the intrinsic atrioventricular delayof the patient is sufficiently long to allow detection of the ends ofintrinsic atrial repolarization events arising following A-pulses, step236, and processing returns to FIG. 2 for determination of preferredAP-VP delay values based upon the ends of the atrial repolarizationevents. If, however, the interval does not exceed the threshold, thenthe intrinsic atrioventricular delay of the patient is not sufficientlylong to allow for detection of the ends of the paced atrialrepolarization events, step 238. Alternative optimization techniques arethen preferably employed to set the AP-VP delay, or the AP-VP delay isset to a default value.

Alternatively, rather than detect the peak of the paced atrialdepolarization event at step 228, the device can simply record the timeat which the A-pulse is delivered. The intrinsic atrioventricular delayfor paced atrial events is then calculating as VS_(PEAK)-A-pulse. Thisinterval is then compared against a suitable detection threshold, whichmay differ from the 260 ms detection threshold used when measuring theintrinsic atrioventricular delay peak-to-peak. Also, as with atrialsensed events, rather than measuring the intrinsic atrioventriculardelay interval based upon the peak of the QRS-complex, the interval mayinstead be measured based on the start of the QRS-complex, though thepeak is preferred for ease and reliability of detection. Also, althoughnot shown in FIG. 3, the intrinsic atrioventricular delay is preferablymeasured for each of a plurality of heartbeats, with the values thenaveraged together before comparison against the minimum thresholdvalues. This is preferred so as to prevent a single anomalousmeasurement of the intrinsic atrioventricular delay from activating ordeactivating the atrial repolarization-based AV optimization techniquesof the invention.

Turning to the FIG. 5, an exemplary technique for determiningpreferred/optimal atrioventricular pacing delay values for use at step204 of FIG. 2 will now be described. Again, processing depends onwhether atrial events are sensed or paced. Referring first to sensedevents, beginning at step 250, the implantable device identifies thepeak of an intrinsic atrial depolarization event (AS_(PEAK)) within theunipolar A-IEGM. At step 252, the end of the subsequent atrialrepolarization event (AT_(END)) is then identified within the unipolarA-IEGM. In one example, a detection window is specified within theA-IEGM signal beginning 100 ms following the peak of the atrialdepolarization event and extending to the end of the intrinsicatrioventricular delay measured at step 214 of FIG. 3. The implanteddevice first detects the atrial T-wave within that window, thenidentifies the end of the atrial T-wave. The end of the T-wave may bedefined as the point at which the slope of the IEGM signal becomessubstantially flat. Otherwise conventional techniques applicable todetecting the ends of ventricular T-waves typically may be employed fordetecting the end of an atrial T-wave. Also, techniques set forth in theabove-identified in patent application of Bruhns et al. may be adaptedfor use in detecting the end point of an atrial T-wave. See, also,end-point detection techniques set forth in U.S. patent application Ser.No. 10/603,398, entitled “System And Method For Detecting CardiacIschemia Based On T-Waves Using An Implantable Medical Device”, of Minet al., filed Jun. 24, 2003, which is incorporated by reference herein.

At step 254, the preferred/optimal AS-VP delay is then determined bycalculating:AS-VP=AS _(PEAK)-A _(TEND)−offset_(SENSED)where offset_(SENSED) is a predetermined offset value set, for example,within the range of 15 to 25 ms and, in one specific example, set to 20ms. Otherwise routine experimentation may be performed for determiningoptimal values for offset_(SENSED) that result in optimization ofcardiac output and/or ventricular filling (or the least result in animprovement therein.)

The preferred/optimal AS-VP delay value is then applied followingsubsequent sensed atrial events to determine the time for deliveringV-pulses to the ventricles. This is illustrated in FIG. 6, whichprovides a stylized representation of a portion of electrical cardiacsignal. During a first heartbeat 256, wherein the ventricles are notpaced, the interval from a peak of the atrial depolarization to the endof atrial repolarization (i.e. AS_(PEAK)-AT_(END)) is measured. (As withexample of FIG. 4, the peak of atrial depolarization event in FIG. 6 isidentified as the first maxima within the event, not the overallmaxima.) An offset is subtracted from the measured interval to yield thepreferred the AS-VP pacing delay. The preferred AS-VP pacing delay isthen applied during a next heartbeat 258 to time delivery of a V-pulsesubsequent to the peak of the atrial depolarization of that heartbeat.By calculating the preferred/optimal the AS-VP pacing delay bysubtracting the offset from AS_(PEAK)-AT_(END), the V-pulse is therebydelivered prior to the end of the atrial T-wave (hence obscuring the endof the atrial T-wave.)

In general, the offset value is preferably set so as to time delivery ofthe V-pulse to be substantially contemporaneous with the atrial T-wave,i.e. the V-pulse is delivered at a point subsequent to the beginning ofthe atrial T-wave but before the end of the atrial T-wave. In theexamples described herein, this is achieved by detecting the end of theatrial T-wave then subtracting the offset value. In otherimplementations, the beginning of the atrial T-wave is instead detectedand then an offset value is added.

Returning to FIG. 5, the processing steps for use with paced atrialevents will now briefly be summarized. Beginning at step 260, theimplantable device identifies the peak of a paced atrial depolarizationevent (AP_(PEAK)) within the unipolar A-IEGM, i.e. the device detectsthe peak of the evoked response triggered by an A-pulse. At step 262,the end of the subsequent atrial repolarization event (AT_(END)) is thenidentified within the unipolar A-IEGM. In one example, a detectionwindow is specified within the A-IEGM signal beginning 100 ms followingthe peak of the atrial depolarization event and extending to the end ofthe paced atrioventricular delay measured at step 232 of FIG. 3. Theimplanted device detects the atrial T-wave within that window thenidentifies the end of the atrial T-wave. At step 234, thepreferred/optimal AP-VP delay is then determined by calculating:AP-VP=AP _(PEAK)-AT _(END)−offset_(PACED)where offset_(PACED) is a predetermined offset value set, for example,within the range of 5 to 15 ms and, in one specific example, set to 10ms. Otherwise routine experimentation may be performed for determiningoptimal values for offset_(PACED) that result in optimization of cardiacoutput and/or ventricular filling (or the least result in an improvementtherein.)

The preferred/optimal AP-VP delay value is then applied followingsubsequent paced atrial events to determine the time for deliveringV-pulses to the ventricles.

Turning to the FIG. 7, an optional technique for adjusting thepreferred/optimal atrioventricular pacing delay values for use at step206 of FIG. 2 will now be described. Preferably, the optimalatrioventricular pacing delay values are more or less continuouslydetected and applied. Hence, if the heart rate changes, new optimalatrioventricular pacing delay values are automatically calculated at thenew rate and hence no rate adjustment to the delay values is required.However, circumstances can arise when it is appropriate to apply delayvalues at a different rate than at rate at which they were initiallyobtained. For example, the ends of atrial repolarizations may beobservable at a base/rest rate, but not at higher rates, therebypreventing optimal delay values from being directly calculated at thehigher rates. If so, then it may be appropriate to rate adjust theoptimal delay values that had been obtained at the base/rest rate foruse at higher rates and FIG. 7 provides an appropriate rate adjustmenttechnique. Again, the particular steps be performed depend upon whetheratrial events are paced or sensed.

For sensed events, step 300 is performed, wherein the AS-VP pacing delayvalue is adjusted as follows:Rate Adjusted AS-VP=β*(AS-VP)where β=current heart rate/rest rate and AS-VP is an optimalatrioventricular delay initially calculated at rest rate.

Hence, AS-VP is adjusted based on the current intrinsic rate of thepatient (which is monitored by the device using otherwise conventionaltechniques), in view of the intrinsic rest rate of the patient (which isperiodically calculated by the device using otherwise conventionaltechniques.)

For paced events in atria, the AP-VP value is adjusted, at step 302, asfollows:Rate Adjusted AP-VP=β*(AP-VP)where β=current pacing rate/base rate and AP-VP is an optimalatrioventricular delay initially calculated at base rate.

Thus, AP-VP is adjusted based on the current pacing rate of the patient,in view of the based pacing rate (which is set by the physician.)

Although rate adjustment has been described with respect to an examplewherein the optimal delay values are initially obtained at rest/baserate, the general rate adjustment technique of FIG. 7 can be adapted toadjust delay values initially obtained at other, higher rates as wellwith appropriate selection of β.

What have been described thus far are various techniques for determiningpreferred or optimal AS-VP and AP-VP delay values for use by animplantable cardiac stimulation device. For the sake of completeness,detailed descriptions of an exemplary implantable cardiac stimulationdevice will now be described.

Exemplary Pacer/ICD

With reference to FIGS. 8 and 9, a description of an exemplary pacer/ICDwill now be provided. FIG. 8 provides a simplified block diagram of thepacer/ICD, which is a dual-chamber stimulation device capable oftreating both fast and slow arrhythmias with stimulation therapy,including cardioversion, defibrillation, and pacing stimulation. Toprovide atrial chamber pacing stimulation and sensing, pacer/ICD 410 isshown in electrical communication with a heart 412 by way of a leftatrial lead 420 having an atrial tip electrode 422 and an atrial ringelectrode 423 implanted in the atrial appendage. Pacer/ICD 410 is alsoin electrical communication with the heart by way of a right ventricularlead 430 having, in this embodiment, a ventricular tip electrode 432, aright ventricular ring electrode 434, a right ventricular (RV) coilelectrode 436, and a superior vena cava (SVC) coil electrode 438.Typically, the right ventricular lead 430 is transvenously inserted intothe heart so as to place the RV coil electrode 436 in the rightventricular apex, and the SVC coil electrode 438 in the superior venacava. Accordingly, the right ventricular lead is capable of receivingcardiac signals, and delivering stimulation in the form of pacing andshock therapy to the right ventricle.

To sense left atrial and ventricular cardiac signals and to provide leftchamber pacing therapy, pacer/ICD 410 is coupled to a CS lead 424designed for placement in the “CS region” via the CS os for positioninga distal electrode adjacent to the left ventricle and/or additionalelectrode(s) adjacent to the left atrium. As used herein, the phrase “CSregion” refers to the venous vasculature of the left ventricle,including any portion of the CS, great cardiac vein, left marginal vein,left posterior ventricular vein, middle cardiac vein, and/or smallcardiac vein or any other cardiac vein accessible by the CS.Accordingly, an exemplary CS lead 424 is designed to receive atrial andventricular cardiac signals and to deliver left ventricular pacingtherapy using at least a left ventricular tip electrode 426, left atrialpacing therapy using at least a left atrial ring electrode 427, andshocking therapy using at least a left atrial coil electrode 428. Withthis configuration, biventricular pacing can be performed. Although onlythree leads are shown in FIG. 8, it should also be understood thatadditional stimulation leads (with one or more pacing, sensing and/orshocking electrodes) might be used in order to efficiently andeffectively provide pacing stimulation to the left side of the heart oratrial cardioversion and/or defibrillation.

A simplified block diagram of internal components of pacer/ICD 410 isshown in FIG. 9. While a particular pacer/ICD is shown, this is forillustration purposes only, and one of skill in the art could readilyduplicate, eliminate or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with cardioversion, defibrillation and pacing stimulation aswell as providing for the aforementioned apnea detection and therapy.

The housing 440 for pacer/ICD 410, shown schematically in FIG. 9, isoften referred to as the “can”, “case” or “case electrode” and may beprogrammably selected to act as the return electrode for all “unipolar”modes. The housing 440 may further be used as a return electrode aloneor in combination with one or more of the coil electrodes, 428, 436 and438, for shocking purposes. The housing 440 further includes a connector(not shown) having a plurality of terminals, 442, 443, 444, 446, 448,452, 454, 456 and 458 (shown schematically and, for convenience, thenames of the electrodes to which they are connected are shown next tothe terminals). As such, to achieve right atrial sensing and pacing, theconnector includes at least a right atrial tip terminal (A_(R) TIP) 442adapted for connection to the atrial tip electrode 422 and a rightatrial ring (A_(R) RING) electrode 443 adapted for connection to rightatrial ring electrode 423. To achieve left chamber sensing, pacing andshocking, the connector includes at least a left ventricular tipterminal (V_(L) TIP) 444, a left atrial ring terminal (A_(L) RING) 446,and a left atrial shocking terminal (A_(L) COIL) 448, which are adaptedfor connection to the left ventricular ring electrode 426, the leftatrial ring electrode 427, and the left atrial coil electrode 428,respectively. To support right chamber sensing, pacing and shocking, theconnector further includes a right ventricular tip terminal (V_(R) TIP)452, a right ventricular ring terminal (V_(R) RING) 454, a rightventricular shocking terminal (V_(R) COIL) 456, and an SVC shockingterminal (SVC COIL) 458, which are adapted for connection to the rightventricular tip electrode 432, right ventricular ring electrode 434, theV_(R) coil electrode 436, and the SVC coil electrode 438, respectively.

At the core of pacer/ICD 410 is a programmable microcontroller 460,which controls the various modes of stimulation therapy. As is wellknown in the art, the microcontroller 460 (also referred to herein as acontrol unit) typically includes a microprocessor, or equivalent controlcircuitry, designed specifically for controlling the delivery ofstimulation therapy and may further include RAM or ROM memory, logic andtiming circuitry, state machine circuitry, and I/O circuitry. Typically,the microcontroller 460 includes the ability to process or monitor inputsignals (data) as controlled by a program code stored in a designatedblock of memory. The details of the design and operation of themicrocontroller 460 are not critical to the invention. Rather, anysuitable microcontroller 460 may be used that carries out the functionsdescribed herein. The use of microprocessor-based control circuits forperforming timing and data analysis functions are well known in the art.

As shown in FIG. 9, an atrial pulse generator 470 and a ventricularpulse generator 472 generate pacing stimulation pulses for delivery bythe right atrial lead 420, the right ventricular lead 430, and/or the CSlead 424 via an electrode configuration switch 474. It is understoodthat in order to provide stimulation therapy in each of the fourchambers of the heart, the atrial and ventricular pulse generators, 470and 472, may include dedicated, independent pulse generators,multiplexed pulse generators or shared pulse generators. The pulsegenerators, 470 and 472, are controlled by the microcontroller 460 viaappropriate control signals, 476 and 478, respectively, to trigger orinhibit the stimulation pulses.

The microcontroller 460 further includes timing control circuitry (notseparately shown) used to control the timing of such stimulation pulses(e.g., pacing rate, atrioventricular delay, atrial interconduction(inter-atrial) delay, or ventricular interconduction (V-V) delay, etc.)as well as to keep track of the timing of refractory periods, blankingintervals, noise detection windows, evoked response windows, alertintervals, marker channel timing, etc., which is well known in the art.Switch 474 includes a plurality of switches for connecting the desiredelectrodes to the appropriate I/O circuits, thereby providing completeelectrode programmability. Accordingly, the switch 474, in response to acontrol signal 480 from the microcontroller 460, determines the polarityof the stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) byselectively closing the appropriate combination of switches (not shown)as is known in the art.

Atrial sensing circuits 482 and ventricular sensing circuits 484 mayalso be selectively coupled to the right atrial lead 420, CS lead 424,and the right ventricular lead 430, through the switch 474 for detectingthe presence of cardiac activity in each of the four chambers of theheart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR. SENSE)sensing circuits, 482 and 484, may include dedicated sense amplifiers,multiplexed amplifiers or shared amplifiers. The switch 474 determinesthe “sensing polarity” of the cardiac signal by selectively closing theappropriate switches, as is also known in the art. In this way, theclinician may program the sensing polarity independent of thestimulation polarity. Each sensing circuit, 482 and 484, preferablyemploys one or more low power, precision amplifiers with programmablegain and/or automatic gain control, bandpass filtering, and a thresholddetection circuit, as known in the art, to selectively sense the cardiacsignal of interest. The automatic gain control enables pacer/ICD 410 todeal effectively with the difficult problem of sensing the low amplitudesignal characteristics of atrial or ventricular fibrillation. Theoutputs of the atrial and ventricular sensing circuits, 482 and 484, areconnected to the microcontroller 460 which, in turn, are able to triggeror inhibit the atrial and ventricular pulse generators, 470 and 472,respectively, in a demand fashion in response to the absence or presenceof cardiac activity in the appropriate chambers of the heart.

For arrhythmia detection, pacer/ICD 410 utilizes the atrial andventricular sensing circuits, 482 and 484, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical signal, and“detection” is the processing of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g., AS, VS, and depolarization signals associated with fibrillationwhich are sometimes referred to as “F-waves” or “Fib-waves”) are thenclassified by the microcontroller 460 by comparing them to a predefinedrate zone limit (i.e., bradycardia, normal, atrial tachycardia, atrialfibrillation, low rate ventricular tachycardia, high rate ventriculartachycardia, and fibrillation rate zones) and various othercharacteristics (e.g., sudden onset, stability, physiologic sensors, andmorphology, etc.) in order to determine the type of remedial therapythat is needed (e.g., bradycardia pacing, antitachycardia pacing,cardioversion shocks or defibrillation shocks).

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 490. The data acquisition system 490 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device502. The data acquisition system 490 is coupled to the right atrial lead420, the CS lead 424, and the right ventricular lead 430 through theswitch 474 to sample cardiac signals across any pair of desiredelectrodes. The microcontroller 460 is further coupled to a memory 494by a suitable data/address bus 496, wherein the programmable operatingparameters used by the microcontroller 460 are stored and modified, asrequired, in order to customize the operation of pacer/ICD 410 to suitthe needs of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude or magnitude, pulse duration, electrodepolarity, rate, sensitivity, automatic features, arrhythmia detectioncriteria, and the amplitude, waveshape and vector of each shocking pulseto be delivered to the patient's heart within each respective tier oftherapy. Other pacing parameters include base rate, rest rate andcircadian base rate.

Advantageously, the operating parameters of the implantable pacer/ICD410 may be non-invasively programmed into the memory 494 through atelemetry circuit 500 in telemetric communication with the externaldevice 502, such as a programmer, transtelephonic transceiver or adiagnostic system analyzer. The telemetry circuit 500 is activated bythe microcontroller by a control signal 506. The telemetry circuit 500advantageously allows intracardiac electrograms and status informationrelating to the operation of pacer/ICD 410 (as contained in themicrocontroller 460 or memory 494) to be sent to the external device 502through an established communication link 504. Pacer/ICD 410 furtherincludes an accelerometer or other physiologic sensor 508, commonlyreferred to as a “rate-responsive” sensor because it is typically usedto adjust pacing stimulation rate according to the exercise state of thepatient. However, the physiological sensor 508 may further be used todetect changes in cardiac output, changes in the physiological conditionof the heart, or diurnal changes in activity (e.g., detecting sleep andwake states) and to detect arousal from sleep. Accordingly, themicrocontroller 460 responds by adjusting the various pacing parameters(such as rate, AS-VP delay, AP-VP delay, V-V delay, etc.) at which theatrial and ventricular pulse generators, 470 and 472, generatestimulation pulses. While shown as being included within pacer/ICD 410,it is to be understood that the physiologic sensor 508 may also beexternal to pacer/ICD 410, yet still be implanted within or carried bythe patient. A common type of rate responsive sensor is an activitysensor incorporating an accelerometer or a piezoelectric crystal, whichis mounted within the housing 440 of pacer/ICD 410. Other types ofphysiologic sensors are also known, for example, sensors that sense theoxygen content of blood, respiration rate and/or minute ventilation, pHof blood, ventricular gradient, etc.

The pacer/ICD additionally includes a battery 510, which providesoperating power to all of the circuits shown in FIG. 9. The battery 510may vary depending on the capabilities of pacer/ICD 410. If the systemonly provides low voltage therapy, a lithium iodine or lithium copperfluoride cell may be utilized. For pacer/ICD 410, which employs shockingtherapy, the battery 510 must be capable of operating at low currentdrains for long periods, and then be capable of providing high-currentpulses (for capacitor charging) when the patient requires a shock pulse.The battery 510 must also have a predictable discharge characteristic sothat elective replacement time can be detected. Accordingly, pacer/ICD410 is preferably capable of high voltage therapy and appropriatebatteries.

As further shown in FIG. 9, pacer/ICD 410 is shown as having animpedance measuring circuit 512 which is enabled by the microcontroller460 via a control signal 514. Thoracic impedance may be detected for usein tracking thoracic respiratory oscillations; lead impedancesurveillance during the acute and chronic phases for proper leadpositioning or dislodgement; detecting operable electrodes andautomatically switching to an operable pair if dislodgement occurs;measuring respiration or minute ventilation; measuring thoracicimpedance for determining shock thresholds; detecting when the devicehas been implanted; measuring respiration; and detecting the opening ofheart valves, etc. The impedance measuring circuit 120 is advantageouslycoupled to the switch 74 so that any desired electrode may be used.

In the case where pacer/ICD 410 is intended to operate as an implantablecardioverter/defibrillator (ICD) device, it detects the occurrence of anarrhythmia, and automatically applies an appropriate electrical shocktherapy to the heart aimed at terminating the detected arrhythmia. Tothis end, the microcontroller 460 further controls a shocking circuit516 by way of a control signal 518. The shocking circuit 516 generatesshocking pulses of low (up to 0.5 joules), moderate (0.5-10 joules) orhigh energy (11 to 40 joules), as controlled by the microcontroller 460.Such shocking pulses are applied to the heart of the patient through atleast two shocking electrodes, and as shown in this embodiment, selectedfrom the left atrial coil electrode 428, the RV coil electrode 436,and/or the SVC coil electrode 438. The housing 440 may act as an activeelectrode in combination with the RV electrode 436, or as part of asplit electrical vector using the SVC coil electrode 438 or the leftatrial coil electrode 428 (i.e., using the RV electrode as a commonelectrode). Cardioversion shocks are generally considered to be of lowto moderate energy level (so as to minimize pain felt by the patient),and/or synchronized with a VS event and/or pertaining to the treatmentof tachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5-40joules), delivered asynchronously (since VS events may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 460 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

In addition, the stimulation device may be configured to performAutomatic Mode Switching (AMS) wherein the pacemaker reverts from atracking mode such as a VDD or DDD mode to a nontracking mode such asVVI or DDI mode. VDD, DDD, WI and DDI are standard device codes thatidentify the mode of operation of the device. DDD indicates a devicethat senses and paces in both the atria and the ventricles and iscapable of both triggering and inhibiting functions based upon eventssensed in the atria and the ventricles. VDD indicates a device thatsensed in both the atria and ventricles but only paces in theventricles. A sensed event on the atrial channel triggers ventricularoutputs after a programmable delay, the pacemaker's equivalent of a PRinterval. WI indicates that the device is capable of pacing and sensingonly in the ventricles and is only capable of inhibiting the functionsbased upon events sensed in the ventricles. DDI is identical to DDDexcept that the device is only capable of inhibiting functions basedupon sensed events, rather than triggering functions. As such, the DDImode is a non-tracking mode precluding its triggering ventricularoutputs in response to sensed atrial events. Numerous other device modesof operation are possible, each represented by standard abbreviations ofthis type.

Insofar as atrioventricular pacing delay values are concerned, themicrocontroller includes an on-board atrial repolarization-based A-VPdelay optimization unit 501, which operates in accordance withtechniques of FIGS. 1 and 7 to determine preferred or optimal AS-VP andAP-VP delay values based on IEGM signals.

In general, while the invention has been described with reference toparticular embodiments, modifications can be made thereto withoutdeparting from the scope of the invention. Note also that the term“including” as used herein is intended to be inclusive, i.e. “includingbut not limited to.”

1. A method for determining atrioventricular pacing delay values for usein delivering cardiac pacing therapy to the heart of a patient in whichan implantable cardiac stimulation device is implanted, the methodcomprising: detecting an atrial repolarization event within anelectrical cardiac signal; and determining a delay value for use by thecardiac stimulation device in pacing the ventricles subsequent to apaced atrial depolarization event by: determining an interval betweenthe paced atrial depolarization event and the atrial repolarizationevent; and subtracting a predetermined offset value (offset_(PACED))from the interval to yield the delay value.
 2. The method of claim 1 andfurther comprising verifying that atrial repolarization events can bedetected within the cardiac electrical signals.
 3. The method of claim 1wherein determining atrioventricular pacing delay values is performed bythe implantable cardiac stimulation device based on IEGM signals.
 4. Amethod for determining atrioventricular pacing delay values for use indelivering cardiac pacing therapy to the heart of a patient in which animplantable cardiac stimulation device is implanted, the methodcomprising: detecting an atrial repolarization event within anelectrical cardiac signal; determining atrioventricular pacing delayvalues for use by the cardiac stimulation device in pacing the heart ofthe patient based on the atrial repolarization event; and adjusting thepacing delay values based on patient heart rate by applying a scalingfactor (β) to an atrioventricular pacing delay value optimized for usewith atrial sensed events at a patient rest rate wherein the scalingfactor is based on current patient intrinsic heart rate and the patientrest rate.
 5. The method of claim 4 wherein the scaling factor (β) iscalculated using:β=(Current Heart Rate)/(Rest Rate).
 6. A method for determiningatrioventricular pacing delay values for use in delivering cardiacpacing therapy to the heart of a patient in which an implantable cardiacstimulation device is implanted, the method comprising: detecting anatrial repolarization event within an electrical cardiac signal;determining atrioventricular pacing delay values for use by the cardiacstimulation device in pacing the heart of the patient based on theatrial repolarization event; and adjusting the pacing delay values basedon patient heart rate by applying a scaling factor (β) to anatrioventricular pacing delay value optimized for use with atrial pacedevents at a base pacing rate wherein the scaling factor is based oncurrent pacing rate and the base pacing rate.
 7. A system fordetermining atrioventricular pacing delay values for use in deliveringcardiac pacing therapy to the heart of a patient in which an implantablecardiac stimulation device is implanted, the system comprising: anatrial repolarization event detector operative to detect an atrialrepolarization event within an electrical cardiac signal; an atrialrepolarization-based atrioventricular pacing delay determination unitoperative to: detect an atrial repolarization event within an electricalcardiac signal; and determine a delay value for use in pacing theventricles subsequent to an atrial depolarization event by determiningan interval between the atrial depolarization event and the atrialrepolarization event; and subtracting a predetermined offset value(offset_(PACED)) from the interval to yield the delay value.
 8. A systemfor determining atrioventricular pacing delay values for use indelivering cardiac pacing therapy to the heart of a patient in which animplantable cardiac stimulation device is implanted, the systemcomprising: means for detecting an atrial repolarization event within anelectrical cardiac signal; means for determining atrioventricular pacingdelay values for use in pacing the heart of the patient based on thedetected atrial repolarization event; means for delivering pacingtherapy using the implantable cardiac stimulation device subject to theatrioventricular delay values; and means for adjusting theatrioventricular pacing delay values based on patient heart ratecomprising means for applying a scaling factor (β) to anatrioventricular pacing delay value optimized for use with atrial eventsat a patient rest rate wherein the scaling factor is based on currentpatient intrinsic heart rate and the patient rest rate.
 9. A method fordetermining atrioventricular pacing delay values for use in deliveringcardiac pacing therapy to the heart of a patient in which an implantablecardiac stimulation device is implanted, the method comprising:detecting an atrial repolarization event within an electrical cardiacsignal; and determining a delay value for use by the cardiac stimulationdevice in pacing the ventricles subsequent to an atrial depolarizationevent by: determining an interval between the atrial depolarizationevent and the atrial repolarization event; and subtracting apredetermined offset value (offset_(PACED)) from the interval to yieldthe delay value.
 10. The method of claim 9 wherein the atrialdepolarization event is a paced atrial event.
 11. The method of claim 9wherein the atrial depolarization event is an intrinsic atrial event.12. A system for determining atrioventricular pacing delay values foruse in delivering cardiac pacing therapy to the heart of a patient inwhich an implantable cardiac stimulation device is implanted, the systemcomprising: an atrial repolarization event detector operative to detectan atrial repolarization event within an electrical cardiac signal; andan atrial repolarization-based atrioventricular pacing delaydetermination unit operative to determine atrioventricular pacing delayvalues for use in pacing the heart of the patient based on the atrialrepolarization event; and adjust the pacing delay values based onpatient heart rate by applying a scaling factor (β) to anatrioventricular pacing delay value optimized for use with atrial eventsat a base pacing rate wherein the scaling factor is based on currentpacing rate and the base pacing rate.